The increasing cost of fossil fuel and environmental concerns have stimulated worldwide interest in developing alternatives to petroleum-based fuels and chemicals. Biomass materials have proven to be a possible renewable alternative for these products.
The production of liquid fuels and chemicals from lignocellulosic materials has received significant attention lately as a non-food feedstock alternative. The most promising of these feedstocks include (1) agricultural wastes, such as bagasse, corn and soybean stover, and straws from wheat, rice, barley, and oats, (2) grasses, such as switch grass, miscanthus, cord grass, and reed canary grass, (3) wood products, such as forest residues, pulping residues, saw dust, and specialty wood crops (e.g., aspen wood), (4) algae-derived biomass, including carbohydrates and lipids from microalgae and macroalgae, and (5) proposed energy crops, such as sweet sorghum.
Lignocellulosic biomass includes three major components. Cellulose—a primary sugar source for bioconversion processes—includes high molecular weight polymers formed of tightly linked glucose monomers. Hemicellulose—a secondary sugar source—includes shorter polymers formed of various sugars. Lignin—the backbone to which cellulose and hemicellulose are bound—is made up of phenylpropanoic acid moieties polymerized in a complex three dimensional structure. The combination of these three components provides a lignocellulosic composition of roughly 30-50% cellulose, 15-35% hemicellulose, and 25-35% lignin, by weight.
Very few cost-effective processes currently exist for efficiently converting cellulose, hemicellulose and lignin to components better suited for producing fuels, chemicals, and other products. This is generally because each of the lignin, cellulose and hemicellulose components demands distinct processing conditions, such as temperature, pressure, catalysts, reaction time, etc., in order to effectively break apart its polymer structure. Due to this distinctness, most processes are only able to convert specific fractions of the biomass, such as the cellulose and hemicellulose components, leaving the remaining components behind for additional processing or alternative uses.
Recent developments involving the catalytic conversion of biomass using aqueous-phase reforming (APR), hydrodeoxygenation (HDO), and other catalytic bioreforming processes, have shown great promise in their ability to convert a wide range of biomass-derived feedstocks to liquid fuels and chemicals. APR and HDO are catalytic reforming processes capable of generating hydrogen and hydrocarbons from oxygenated hydrocarbons without a significant disruption of the carbon backbone. The oxygenated hydrocarbons may include starches, mono- and poly-saccharides, sugars, sugar degradation products, and sugar alcohols, as well as other polyols, organic acids, furfurals, phenols, cresols and other degradation products typically produced by lignocellulosic deconstruction technologies.
Various APR methods and techniques are described in U.S. Pat. Nos. 6,699,457; 6,964,757; 6,964,758; and 7,618,612 (all to Cortright et al., and entitled “Low-Temperature Hydrogen Production from Oxygenated Hydrocarbons”); U.S. Pat. No. 6,953,873 (to Cortright et al., and entitled “Low-Temperature Hydrocarbon Production from Oxygenated Hydrocarbons”); and U.S. Pat. Nos. 7,767,867 and 7,989,664; and U.S. Patent Application Ser. No. 2011/0306804 (to Cortright, and entitled “Methods and Systems for Generating Polyols”). Various APR and HDO methods and techniques are described in U.S. Pat. Nos. 8,053,615; 8,017,818; and 7,977,517; and U.S. Patent Application Ser. Nos. 2011/0257448; 2011/0245543; 2011/0257416; and 2011/0245542 (all to Cortright and Blommel, and entitled “Synthesis of Liquid Fuels and Chemicals from Oxygenated Hydrocarbons”); U.S. Patent Application Ser. No. 2009/0211942 (to Cortright, and entitled “Catalysts and Methods for Reforming Oxygenated Compounds”); U.S. Patent Application Ser. No. 2010/0076233 (to Cortright et al., and entitled “Synthesis of Liquid Fuels from Biomass”); International Patent Application No. PCT/US2008/056330 (to Cortright and Blommel, and entitled “Synthesis of Liquid Fuels and Chemicals from Oxygenated Hydrocarbons”); and commonly owned co-pending International Patent Application No. PCT/US2006/048030 (to Cortright et al., and entitled “Catalyst and Methods for Reforming Oxygenated Compounds”), all of which are incorporated herein by reference.
The first step in the process of converting lignocellulosic biomass to a useable feedstock involves the deconstruction of the complex polymeric matrices of the cellulosic, hemi-cellulosic and lignin material. This is typically accomplished using the following methods, either alone or in combination: (1) thermochemical treatment using a mineral acid, strong base, water at autohydrolysis conditions, gas catalyst, oxidation catalyst, and/or an organic solvent (2) enzymatic hydrolysis, and more recently (3) catalytic biomass deconstruction.
Acid hydrolysis is one type of thermochemical treatment. In acid hydrolysis, the feedstock is subjected to steam and a mineral acid (e.g., sulfuric acid, hydrochloric acid, or phosphoric acid) to hydrolyze the cellulose and hemicellulose to their monomeric components. For cellulose, this is glucose, while hemicellulose is hydrolyzed to xylose, galactose, mannose, arabinose, acetic acid, galacturonic acid, and glucuronic acid. Sulfuric acid, hydrochloric acid, and phosphoric acid are the three most common mineral acids used for this process. Once complete, the resulting slurry contains the mineral acid, as well as residual or unreacted fiber from lignin, and an aqueous solution of the desired sugars and other hydrolysate products, such as organic acids, including primarily acetic acid, but also formic acid, propionic acid, malic acid, citric acid, oxalic acid, lactic acid, butyric acid, valeric acid, aconitic acid, caproic acid, 2-furoic acid, vanillic acid, syringic acid, protocatechuic acid, ferulic acid, p-coumaric acid, sinapic acid, gallic acid, glucuronic acid, galacturonic acid, cellobiouronic acid, aldonic acids, aldaric acids, hexanoic acid, heptanoic acid, salts, and other degradation products, including furfurals, phenols, and cresols. The cost of fresh mineral acid is a significant operational cost for acid hydrolysis of biomass.
Enzymatic hydrolysis typically involves a thermochemical pretreatment followed by hydrolysis with cellulose enzymes. The thermochemical pretreatment is used to increase the surface area of the cellulose material to allow enzyme penetration. When compared to acid hydrolysis alone, the thermochemical pretreatment steps are generally milder (e.g., lower mineral acid concentrations, shorter treatment times, etc.). After acid pretreatment, base is added to the solution to raise the pH to a range in which the enzyme is active and, in the process, the mineral acid is converted into a mineral salt. Similar to the acid hydrolysis process, the hemicellulose is hydrolyzed by the mineral acid to xylose, galactose, mannose, arabinose, acetic acid, galacturonic acid, and glucuronic acid. The cellulose is hydrolyzed by the enzymes to glucose. The cost of enzymes and base are significant operational costs for enzymatic hydrolysis of biomass.
Catalytic biomass deconstruction involves the use of a heterogeneous catalyst to hydrolyze the cellulose, hemicellulose and, in some instances, the lignin to water-soluble oxygenated hydrocarbons. The oxygenated hydrocarbons include carbohydrates, starches, polysaccharides, disaccharides, monosaccharides, sugars (including glucose, xylose, galactose, mannose, arabinose), sugar degradation products (e.g., hydroxymethyl furfural (HMF), levulinic acid, formic acid, and furfural), sugar alcohols, alditols, polyols, diols, alcohols, ketones, cyclic ethers, esters, carboxylic acids, aldehydes, phenols, cresols and other oxygenated hydrocarbon species.
Regardless of the deconstruction process used, the resulting hydrolysate stream is likely to contain the desired sugars, organic acids and other oxygenated hydrocarbons derived from hemicellulose, cellulose, and lignin, as well as contaminants, such as mineral salts, mineral acids, other solvents used in deconstruction, terpenoids, stilbenes, flavonoids, proteinaceous materials, metal impurities, ash and other organic products. The glucose can be readily converted to butanol using bacteria or ethanol using conventional yeast fermentation techniques. The pentose sugars may also be converted to a wide variety of fuels and chemicals using recombinant yeast, bacteria, or algae. The remaining organic and inorganic materials may be used for fertilizer or other applications, while the mineral acids and mineral salts may be recycled for continued use in the purification system.
The presence of the organic acids, their corresponding salts and other oxygenated hydrocarbons in a hydrolysate stream are deleterious to fermentation and other biological processes. In particular, the presence of these compounds can inhibit effective yeast, bacterial, or algal activity, resulting in reduced yields or a corresponding increase in the amount of yeast, bacteria, algae or enzymes required. It is therefore imperative for fermentation and other biological processes to remove organic acids and other non-sugar compounds to produce a feedstock stream containing primarily sugar. The result is a reduction in overall product yield from the biomass due to the removal of the organic acids and other oxygenated hydrocarbons from the fuel conversion process.
The APR and HDO technologies, on the other hand, are able to convert mixed feedstock streams containing not only sugars and sugar alcohols, but also organic acids, and other oxygenated hydrocarbons. Contaminants, such as mineral salts, mineral acids, proteinaceous materials, ash and other organic products, however, can decrease catalyst lifetime and functionality. It is therefore desirable to remove these contaminants and provide a feedstock stream containing the sugars, as well as the organic acids and other oxygenated hydrocarbons derived from the lignocellulosic conversion process.